This invention particularly relates to a solid nitrogen gas generating composition useful as a nitrogen source for inflating an inflatable occupant restraint used to protect passengers in an automobile subjected to severe impact. Inflatable restraints are generally regarded as a preferred means for cushioning the impact of a passenger against the interior of the automobile and are especially effective when utilized in conjuction with safety belts. It is preferred that a solid gas generating composition be used as the source of the gas, because the volume required for storage of the solid is small, no high pressure container is required, and, desired characteristics of gas generation are more easily tailored for a solid composition. Moreover, a solid may be maintained in predictably good operating condition over an extended period of time with minimal expense, compared with a gas generating composition in any other form.
The many strick requirements of a solid gas generator composition for an inflatable restraint have been enumerated nearly as often as inflatable restraints have been discussed. For example, it is well known that a non-toxic gas must be generated in less than about 60 milliseconds in a large enough quantity to provide the necessary inflation, yet without destroying the bag. The temperature of the gas grenerated must be low enough so as not to burn the bag and inflict series injury on passengers who have been spared severe impact within the automobile.
Though the prior art is replete with numerous gas generating compositions, and particularly azide containing compostions to generate nitrogen, no gas generating composition has been suggested which yields upon ignition, a solid porous coherent sinter, hereinafter referred to simply as "sinter". By the term "sinter" I further describe a fused combustion residue which may be tailored for desirable physical and chemical characteristics, and predictably derived from a desirable nitrogen gas generating composition which fulfills the exacting requirements for an inflatable restraint. Formation of a porous sinter provides built-in self-filtration of products of combustion, and, for the relatively few particles which do attempt to escape, a simple retention system. The porous sinter reduces the stringency of demands imposed upon sophisticated filtration devices for confining explosively propelled particles of the combustion residue.
In particular, a prior art gas generating composition for inflating an inflatable confining means or occupant restraint is disclosed in German Offenlegugsschrift No. 2,325,310 laid open Dec. 6, 1973 wherein a gas generating solid mixture contains at least one substance which represents an alkaline earth metal azide, alkali metal azide or hydroxy metal azide of the general formula M(OH).sub.m (N.sub.3).sub.n in which M stands for magnesium, calcium, strontium, zinc, boron, aluminum, silicon, tin, titanium, zirconium, manganese, chromium, cobalt or nickel, m and n the valence of the atoms M, and m and n each time signify a whole number, as well as at least one oxidation agent and/or a combustible mixture which includes at least one oxidation agent and/or a reduction agent. Strontium azide is specifically preferred over alkali metal azides and particularly over sodium azide, because strontium azide is more easily decomposed, because of its lower decomposition temperature, and its smaller activation energy for decomposition. It is further stated that, where strontium azide is used, potassium perchlorate must be added in a quantity of about 5 percent by weight in relation to the quantity of strontium azide. Though, surprisingly alkaline earth metal azides are not known to form a coherent sinter when used as reactants in combination with the oxidation agents identified in the aforementioned German reference, more surprisingly, potassium perchlorate is not an essential ingredient in the gas generating composition of my invention. Among the oxidation agents disclosed in the aforementioned reference are various perchlorates, nitrates, metal peroxides, and metal oxides including ferric oxide, ferrous oxide and ferroso ferric oxide. The disclosed gas generating composition is contained in a chamber enclosed by a filtration wall composed mainly of several layers of closely woven metal wire gauze designed to trap finely divided particles of combustion residue. Specifically, the examples disclose that, upon ignition, essentially all the solid nitrogen gas generating composition is converted to a finely divided combustion residue, and, essentially all of this residue is trapped in the finely woven metal wire gauze layers fastened in the upper portion of a container. The gas generating composition was placed in the bottom of the container. Other examples reiterate that essentially all the solid gas generating composition is explosively converted to liquid and no coherent sinter is left.
Another prior art composition disclosed in U.S. Pat. No. 3,741,585 includes an alkali metal azide, a metallic sulfide, certain metallic oxides and sulphur to produce nitrogen at a temperature in the range from about 200.degree. to about 1000.degree. F. Metallic oxides disclosed are the oxides of molybdenum, tungsten, lead and vanadium. There is no indication as to the manner in which the combustion residue is contained nor of the physical form in which it is obtained.
To the best of my knowledge the prior art compositions do not yield, upon ignition, a solid, coherent, porous combustion residue. Instead, known compositions yield a fine hot powder of combustion residue particles, or liquid, which are carried in the gaseous product.